Calcium phosphates (CaP) are being investigated for a variety of medical applications ranging from tissue engineering scaffolds to nonviral gene carriers. Examples of prior methods for the formation of calcium phosphate nanoparticles include precipitation techniques, hydrothermal processing, solid state reactions, molten salt synthesis, and microwave-assisted techniques. Chemical synthesis methods are better than traditional solid-state methods in producing nanocrystalline calcium phosphates since solid-state methods often require mechanical mixing of oxides or carbonates after calcinations as well as longer synthesis times. This causes the nanoparticles produced to be of an inferior quality, as observed by their poor sintering behavior, non-homogeneity, and abnormal grain growth.
There is also difficulty in having precise control of the cation stoichiometry. Conversely, chemical processing routes diminish deficiencies associated with diffusion, introduction of impurities, and particle agglomeration. The chemical composition of the particles as well as their morphology and crystallinity are important considerations for medical applications.
In addition, the chemical composition of the material greatly impacts its biocompatibility, bioactivity, and biodegradability. Each calcium phosphate phase has a unique set of attributes due to its crystal structure and availability of calcium and phosphate content. These features greatly impact the manner in which the material will interact with biological systems.
Hydroxyapatite (HA) is the most commonly synthesized calcium phosphate phase, owing to its stability in aqueous solutions and ease of synthesis. HA is also the most similar CaP phase to mammalian bone. While tricalcium phosphate (TCP) is more biodegradable than HA, TCP is more difficult to synthesize, mainly due to the fact that it forms at elevated temperatures and there is a narrow range of parameters at which TCP will be the most stable phase. The majority of synthesis methods currently used to produce TCP nanoparticles are two step processes: precipitation of a precursor, followed by high temperature sintering. It is difficult to transform their morphology into elongated nanoparticles through conventional heating.